Reading posts on various boards here (Boomers presently!) about the frustrations of budding script writers on this site here's my pennyworth.
I gave up sit- com script writing years ago as I'd have enough of seemingly arbitrary rejection and double standards.
I then tried sketch writing with moderate success, BUT THEN saw a couple of my rejected sketches turn up a year later on the same show - talk about spitting nails!!!!!!!!
The final nail in the coffin for me was when I regularly sent in sketches for Hale and Pace and was told in no uncertain terms that they NEVER did a sketch where one of them was dressed as a woman (I'd sent one in of course), and what happened? That's right - that same series had a sketch where Gareth was in female attire.
As I say, double standards and the other thing that used to grate is that the script editors also sometimes "submit" their own work, which of course is accepted. I am sure that my rejected Hale & Pace sketches were plagiarised by the script editor. How that is fair I don't know.
Who assesses and edits the editors? No one it seems.
Best of "luck" to all of you that are still banging at that door - for me they can stuff it where the sun don't shine. 
I see nothing in the above to show any kind of qualification to be doing the job you fancy doing. Maybe address that in ways you can?
